Web entrepreneur Matt Lombardi said he just wanted to get into shape -- but it took only one session of the popular P90X workout series to send the 29-year-old to the hospital with rhabdomyolysis, a relatively rare condition he said his doctors told him occurs when muscle tissue breaks down to such an extent that toxins released into the blood stream can damage the kidneys.

"I was so sore I couldn't lift my arms and then I noticed my pee was the color of cola," Lombardi recalled. "After Googling P90X and my symptoms, I immediately made an appointment with a kidney specialist."

Fortunately, after a five-day hospital stay, the problem subsided and doctors told Lombardi there probably wouldn't be any lingering effects. However, Lombardi's experience may underscore the hazards of high-intensity interval training, or HIITS, an exercise and weight-loss trend that involves alternating bouts of extremely vigorous exercise with brief rest periods.

HIITs programs have exploded in popularity in the past few years. The P90X DVD series has sold more than 4.2 million copies, according to Beach Body, the company that distributes the program. Insanity, also by Beach Body, has sold more than 2 million copies. Cross Fit, another popular HIITs workout system, has, according to its website, more than 5,000 affiliate gyms in North America with thousands more on other continents.

Research suggests that HIITs may be a superior way to slim down and muscle up compared to the slow and steady "fat burning" workout style that has prevailed since the 1980s. For example, a recent study out of Australia found women who followed a 20-minute HIITs program lost six times more body fat than women who followed a 40-minute moderate-intensity cardio program.

In another recent study, Michele Olson, a professor at Auburn University in Alabama, determined that a HIITs-style method known as Tabata blasted an average of 13.5 calories per minute compared to a mere 6 to 8 calories per minute burned in the typical cardio program.

This is only part of the reason high-intensity workouts are so effective for weight loss, she said.

"High-intensity exercise also has the advantage of elevating your metabolic rate post-exercise for a period of time after you stop working out so you continue to burn fat and calories at a higher rate for a long time afterwards," Olson said. "Overall, it would take five times the amount of typical cardio exercise to shed the same number of calories you can in a four-minute Tabata."

Olson said that high-intensity training also does a better job strengthening the heart, increasing aerobic capacity and improving blood levels of fat, cholesterol and glucose.

However, Steve Edwards, the vice president of fitness and nutrition at Beach Body, said that despite marketing that depicts models who go from fat to fit by doing the company's workouts, programs like P90X were never intended for beginning exercisers.

"These were always meant to be 'graduate programs' from some of our other workouts," he said. "Our target audience may have been deconditioned when they started with us, but for P90X, we see them as someone who has been dedicated to another program for awhile and aspires to go further.

"This kind of workout was never intended for someone who is grossly overweight and who has been rooted to the couch for years," he added.

Edwards pointed out that Beach Body programs contain explicit statements warning exercisers to check with their doctors and not overdo it. They are encouraged to take the fitness test that comes with their program that can help them evaluate whether or not they are ready. If they're not, they can ask for a refund, he said.

He added that Beach Body instructors demonstrate several different versions of the exercises within the workouts to accommodate the less fit and those struggling with problems like bum knees and sore backs.

Experts said problems can arise when exercisers try to do too much, too soon.

Dr. Stephen Fealy, an orthopedic surgeon with the Hospital for Special Surgery in New York, said he started seeing a spike in HIITs-related injuries two years ago. Now, at least one patient a week who has overdone it limps into his office.

Rather than traumatic injuries like broken bones and stress fractures, he said, high-intensity exercisers tend to rack up muscle sprains and tendon strains, particularly of the calf, chest and shoulder, the result of overusing explosive movements and heavy weights.

"I think these programs are quite good, but if someone goes from couch to full throttle without any preparation, there's a good chance they're going to get hurt," he said.

As for reports of cases like Lombardi's rhabdomyolysis or other serious medical consequences, Cedric Bryant, the chief science officer for the American Council on Exercise in San Diego, noted that they have been few and far between.

"Most people naturally back off before they get to that point because they don't have the capacity to push themselves that far," he said. "Because most people starting out are less than fit, they may perceive they are really going for it but, physiologically, they have to stop long before they can get into this kind of trouble."

Bryant said anyone can do some kind of damage to their body if they jump into any sort of fitness routine without proper training, but people like Lombardi -- who claimed to be in decent shape before he tried the workout -- are at greater risk for doing serious harm because they can physically perform at a level that someone who is out of shape and overweight can't.

Richard Joe, who owns a Cross Fit gym on the Upper East Side of New York City, said the goal of Cross Fit is to be able to complete the workouts, which can be super intense, but not from day one.

"The best way to do Cross Fit is to work with an experienced coach who can help you set reasonable goals for you to gradually work towards," he said.

Bryant said he is a fan of high-intensity workouts because they can be time- and budget-conscious -- and because they often get results where other workouts may not. He did, however, stress how important it is for newbies to "listen to their body" and work their way into a HIITs program by doing some moderately paced exercise for a month or two.

"You must be smart enough to respect your physical boundaries," he said.

Lombardi, who said he's done with heavy lifting programs, agreed.

"I don't blame the workout -- but I do want people to know this can happen," he said.

Have you tried a HIITs style program? What was your experience? Feel free to sound off in the comments section below.